Scientists have long-believed if temperatures dropped to absolute zero molecules would stop behaving as individuals and move as one collective body, creating exotic states of matter that have never been observed in the physical world. Now, a research team has successfully cooled molecules to almost absolute zero to make some fascinating findings.
The researchers found ultracold molecules were relatively long-lived, stable, and rarely collided with each other, MIT reported. They exhibited strong dipole moments, which are imbalances in the electric charges within molecules that mediate the magnet-like forces between them. The chilly molecules also proved to slow down their rotations dramatically.
"We are very close to the temperature at which quantum mechanics plays a big role in the motion of molecules," said Martin Zwierlein, professor of physics at MIT and a principal investigator in MIT's Research Laboratory of Electronics. "So these molecules would no longer run around like billiard balls, but move as quantum mechanical matter waves. And with ultracold molecules, you can get a huge variety of different states of matter, like superfluid crystals, which are crystalline, yet feel no friction, which is totally bizarre. This has not been observed so far, but predicted. We might not be far from seeing these effects, so we're all excited."
The team used lasers and evaporative cooling to cool clouds of individual sodium and potassium atoms to near absolute zero, and "glued" the atoms together to form ultracold molecules by applying magnetic fields in a process called "Feshbach resonance."
"It's like tuning your radio to be in resonance with some station," Zwierlein said. "These atoms start to vibrate happily together, and form a bound molecule."
To make this bond stronger, the researchers exposed the molecules to a pair of lasers that matched both their highest and lowest possible vibrational state. The absorption of the low-energy laser and emissions from the high-energy beam caused the molecules to lose all of their vibrational energy.
"In terms of temperature, we sucked away 7,500 kelvins, just like that," Zwierlein said.
Past research has shown ultracold potassium rubidium molecules were chemically reactive and came apart when they collided with one another. The researchers chose to create ultracold molecules of sodium potassium, because they are chemically stable and resilient against these collisions.
"When two potassium rubidium molecules collide, it is more energetically favorable for the two potassium atoms and the two rubidium atoms to pair up," Zwierlein said. "It turns out with our molecule, sodium potassium, this reaction is not favored energetically. It just doesn't happen."
The team observed the molecular gas they created was stable and long-lived, as in they lasted for about 2.5 seconds.
"In the case where molecules are chemically reactive, one simply doesn't have time to study them in bulk samples: They decay away before they can be cooled further to observe interesting states," Zwierlein said. "In our case, we hope our lifetime is long enough to see these novel states of matter."
By cooling the atoms and then forming them into molecules, the researchers created an ultracold gas of molecules that measured up to one thousand times cooler than what is possible through direct cooling techniques.
The findings were published in a recent edition of the journal Physical Review Letters.
